If portable is meant to describe the drive and not what it's connected to, then I have a Lacie Porsche 80GB FW400/USB drive. It's great for taking along on road trips or use as a bootable clone backup of the internal laptop drive. It's a little small and a little slow these days, and I am eyeing one of the newer Lacie rugged drives with FW800.
If we're talking external storage, then I have two Maxtor 300GB 3Gbps SATA drives in USB/eSATA enclosures. They normally live attached to my AEBS as airdisks, but if I need a big fat data pipe then I connect them directly to my MBP via a Sonnet Technologies eSATA expresscard.
If we're talking external storage, then I have two Maxtor 300GB 3Gbps SATA drives in USB/eSATA enclosures. They normally live attached to my AEBS as airdisks, but if I need a big fat data pipe then I connect them directly to my MBP via a Sonnet Technologies eSATA expresscard.